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~ ..•.. There are two types of Oxford Filter; blue filters enhance short wavelength light and yel,low enhance long wavelength light. These filters have previously been shown to improve reading performance, vergence eye movements and perception of visual form and motion. In this thesis the effects of such filters were examined in both an unselected primary school population, and in a clinical population of children with identified visual and/or reading difficulties. A school based visual screening study established that just over 40% of children identified a filter as beneficial when viewing text. These children reported a significantly higher number of visual symptoms and had significantly reduced convergence and accommodation eye movements compared to non-filter choosers. A further trial of filter use revealed that those using blue filters made significant improvements in accommodative function and in spelling ability. A cross-over, randomised controlled trial was conducted in the clinical sample with psychometric, orthoptic and psychophysical assessments administered before and after filter use. Poor readers made significant improvements in reading following filter use, but a smaller dyslexic group were not found to improve their reading. Performance on a Matrices task significantly improved after using yellow filters specifically. There was also evidence that children with reduced saccades improved their spelling with the use of yellow filters, but had reduced spelling following blue use. Although filters impacted upon both orthoptic and literacy measures these improvements appear not to be causally linked. A random dot kinematogram task (RDK) revealed a correlation between magnocellular functioning under blue and yellow lighting conditions and reading; lower reading was associated with poorer RDK thresholds. This association was specific to reading and not evident in relation to dyslexia. Visual search accuracy was also shown to improve significantly after the use of blue and yellow filters. Together these findings have implications for the treatment of orthoptic abnormalities and literacy performance. |
